Deep-space optical communication links operate under severely limited signal power, approaching the photon-starved regime that requires a receiver capable of measuring individual incoming photons. This makes the photon information efficiency (PIE), i.e., the number of bits that can be retrieved from a single received photon, a relevant figure of merit to characterize data rates achievable in deep-space scenarios. We review theoretical PIE limits assuming a scalable modulation format, such as pulse position modulation (PPM), combined with a photon counting direct detection receiver. For unrestricted signal bandwidth, the attainable PIE is effectively limited by the background noise acquired by the propagating optical signal. The actual PIE limit depends on the effectiveness of the noise rejection mechanism implemented at the receiver, which can be improved by the nonlinear optical technique of quantum pulse gating. Further enhancement is possible by resorting to photon number resolved detection, which improves discrimination of PPM pulses against weak background noise. The results are compared with the ultimate quantum mechanical PIE limit implied by the Gordon–Holevo capacity bound, which takes into account general modulation formats as well as any physically permitted measurement techniques.

中文翻译：

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深空光通信中的光子信息效率限制

深空光通信链路在严格有限的信号功率下运行，接近光子匮乏状态，需要能够测量单个传入光子的接收器。这使得光子信息效率（PIE），即可以从单个接收光子中检索到的位数，成为表征深空场景中可实现的数据速率的相关品质因数。我们回顾了假设可扩展调制格式（例如脉冲位置调制 (PPM)）与光子计数直接检测接收器相结合的理论 ​​PIE 限制。对于不受限制的信号带宽，可达到的 PIE 受到传播光信号获取的背景噪声的有效限制。实际的 PIE 限制取决于接收器处实施的噪声抑制机制的有效性，可以通过量子脉冲选通的非线性光学技术来改进该机制。通过采用光子数分辨检测可以进一步增强，这可以提高 PPM 脉冲对弱背景噪声的辨别能力。将结果与 Gordon-Holevo 容量界限所暗示的最终量子力学 PIE 极限进行比较，该极限考虑了通用调制格式以及任何物理允许的测量技术。